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I'm new here and a beginner level programmer in C. I'm having some problem with using openmp to speedup the for-loop. Below is simple example: #include <stdlib.h> #include <stdio.h> #include <gsl/gsl_rng.h> #include <omp.h> gsl_rng *rng; main() { int i, M=100000000; double tmp; /* initialize RNG */ gsl_rng_env_setup(); rng = gsl_rng_alloc (gsl_rng_taus); gsl_rng_set (rng,(unsigned long int)791526599); // option 1: parallel #pragma omp parallel for default(shared) private( i, tmp ) schedule(dynamic) for(i=0;i<=M-1;i++){ tmp=gsl_ran_gamma_mt(rng, 4, 1./3 ); } // option 2: sequential for(i=0;i<=M-1;i++){ tmp=gsl_ran_gamma_mt(rng, 4, 1./3 ); } } The code draws from a gamma random distribution for M iterations. It turns out the parallel approach with openmp (option 1) takes about 1 minute while the sequential approach (option 2) takes only 20 seconds. While running with openmp, I can see the cpu usage is 800% ( the server I'm using has 8 CPUs ). And the system is linux with GCC 4.1.3. The compile command I'm using is gcc -fopenmp -lgsl -lgslcblas -lm (I'm using GSL ) Am I doing something wrong? Please help me! Thanks! P.S. As pointed out by some users, it might be caused by rng. But even if I replace tmp=gsl_ran_gamma_mt(rng, 4, 1./3 ); by say tmp=1000*10000; the problem still there...

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